Concentric
Two geometric figures are said to be concentric if their centers coincide. The region between two concentric circles is called an annulus. The following table summarizes some concentric central circles. Kimberling | center | circles X_1 | incenter I | Adams' circle, Conway circle, hexyl circle, incircle X_2 | triangle centroid G | inner Napoleon circle, outer Napoleon circle X_3 | circumcenter O | circumcircle, second Brocard circle, second Droz-Farny circle, Stammler circle X_4 | orthocenter H | anticomplementary circle, polar circle, first Droz-Farny circle X_5 | nine-point center N | nine-point circle, Steiner circle X_10 | Spieker center S p | excircles radical circle, Spieker circle X_39 | Brocard midpoint | Gallatly circle, half-Moses circle, Moses circle X_182 | midpoint of the Brocard diameter | Brocard circle, first Lemoine circle
